Integrated electrical circuits and other microstructured components are conventionally produced by applying a plurality of structured layers onto a suitable substrate which, for example, may be a silicon wafer. In order to structure the layers, they are first covered with a photoresist which is sensitive to light of a particular wavelength range, for example light in the deep ultraviolet (DUV) spectral range. The wafer coated in this way is subsequently exposed in a projection exposure apparatus. A pattern of diffracting structures, which is arranged on a mask, is thereby imaged onto the photoresist with the aid of a projection objective. Since the imaging scale is generally less than 1, such projection objectives are often also referred to as reducing objectives.
After the photoresist has been developed, the wafer is subjected to an etching process so that the layer becomes structured according to the pattern on the mask. The remaining photoresist is then removed from the other parts of the layer. This process is repeated until all the layers have been applied onto the wafer.
The performance of the projection exposure apparatus being used is determined not only by the imaging properties of the projection objective, but also by an illumination system which illuminates the mask. To this end, the illumination system contains a light source, for example a laser operated in pulsed mode, and a plurality of optical elements which generate light bundles, converging on the mask at field points, from the light generated by the light source. The individual light bundles have particular properties, which in general are adapted to the projection objective.
These properties include inter alia the illumination angle distribution of the light bundles which respectively converge on a point in the mask plane. The term illumination angle distribution describes the way in which the overall intensity of a light bundle is distributed between the different directions in which the individual rays of the light bundle strike the relevant point in the mask plane. If the illumination angle distribution is specially adapted to the pattern contained in the mask, then the latter can be imaged with high imaging quality onto the wafer covered with photoresist.
The illumination angle distribution is often described not directly in the mask plane, in which the mask to be projected is placed, but instead as an intensity distribution in a generalised pupil or a real pupil surface, which has a Fourier relation with the mask plane. The generalised pupil corresponds to an exit pupil lying at infinity, whereas a real pupil surface is distinguished in that the principal rays of the ray bundles intersect the optical axis there. Description of the illumination angle distribution (angle space) with the aid of an intensity distribution (position space) utilises the fact that each angle with respect to the optical axis, at which a light ray passes through a field plane, can be assigned a radial distance measured from the optical axis in a Fourier-transformed pupil surface. Similar considerations also apply to a generalised pupil. For the sake of simplicity, in what follows the description of the illumination angle distribution will refer to the intensity distribution in a real pupil surface, unless expressly indicated otherwise.
The illumination angle distribution, or the intensity distribution equivalent thereto in a pupil surface or idealised exit pupil, will sometimes also be referred to as an “illumination setting”. Depending on the basic shape of the intensity distribution, distinction is made between so-called conventional and unconventional illumination settings.
In the case of conventional illumination settings, the intensity distribution in a pupil surface has the shape of a circular disc concentric with the optical axis. Each point in the mask plane is therefore struck by light rays at angles of incidence of between 0° and a maximum angle dictated by the radius of the circular disc. In the case of unconventional illumination settings, which include in particular ring-field illuminations and multi-pole illuminations, the region illuminated in the pupil surface has the shape of a ring concentric with the optical axis, or a plurality of individual regions (poles) which are arranged at a distance from the optical axis. With these settings, therefore, the mask to be projected is only illuminated obliquely. In the case of dipole illumination, for example, two poles are arranged on a radius with an equal radial distance from the optical axis. The intensity distribution for quadrupole illumination is obtained as a superposition of two dipole illuminations mutually rotated by 90°.
The operators of projection exposure apparatus generally establish a target illumination angle distribution, which is regarded as particularly suitable for imaging a particular mask optimally with the aid of a projection objective onto a photoresist or another photosensitive layer. The task is then to adjust the illumination system so that the target illumination distribution is achieved with the greatest possible accuracy in the mask plane. To this end, illumination systems generally include a plurality of adjustable optical elements whose optical effect on the illumination angle distribution can be modified as a function of control commands. These optical elements may for example be replaceable diffractive optical elements, zoom objectives and/or pairs of axicon elements. Additional measures are furthermore often provided, by which fine tuning or correction of the illumination angle distribution can be achieved. In this context, for example, U.S. Pat. No. 6,535,274 B2 proposes adjustable transmission filters which are arranged in or in the immediate vicinity of the pupil surface.
The greater the number of optical elements, whose effect on the illumination angle distribution can be modified as a function of control commands, the more difficult it is to determine the control commands so that the illumination angle distribution actually set up corresponds as much as possible to the target illumination angle distribution. This problem is encountered above all when new types of illumination angle distributions are intended to be set up, for which there is not yet any experience as to the control commands with which the new type of illumination angle distribution can be achieved.
To date, attempts have been made to describe the illumination angle distribution for the various illumination settings with the aid of relatively simple models. Various quantities have been defined, which can be deliberately influenced with the aid of particular optical elements, for example filter elements arranged near the pupil. In the case of conventional, annular or quadrupole illumination settings, for example, a quantity referred to as the pupil ellipticity is often employed. Simply speaking, the pupil ellipticity corresponds to the ratio of the amounts of light which strike a field point on the mask from orthogonal directions during exposure. The greater the pupil ellipticity deviates from 1, the more asymmetric is the illumination angle distribution.
Another quantity, which has to date been employed for describing illumination angle distributions, is the telecentricity. Illumination is referred to as energetically telecentric when the energetic central rays of the light bundles, which are often also referred to as centroid rays, pass perpendicularly through the mask plane. Similar considerations apply to the geometrical telecentricity, for which it is not the energetic but the geometrical central rays (i.e. principal rays) of the light bundles which are considered.
In the case of energetically non-telecentric illumination, the entire light bundles strike the mask to some extent obliquely. For the illumination angle distribution, this means that the amounts of light coming from opposite directions are of different size. In general, telecentric illumination is desired since the projection objectives are usually likewise telecentric on the object side.
Such quantities, however, sometimes cannot be used expediently for all illumination settings. Above all, these quantities generally do not allow the intensity distribution in the pupil surface (and therefore the illumination angle distribution in the mask plane) to be described so accurately that the intensity distribution could be reconstructed based on these quantities.
An article by T. Heil et al. entitled “Predictive modeling of advanced illumination pupils used as imaging enhancements for low-k1 applications”, Optical Microlithography XVII, B. W., Smith, ed., Proc SPIE 5377, pages 344-356, SPIE, March 2004 discloses a parametric pupil model with which annular illumination settings can be described very precisely. With the aid of a relatively limited number of pupil parameters, it is possible to describe ring-shaped intensity distributions in a pupil surface so accurately that the intensity distribution can be reconstructed merely based on the illumination parameters. This article does not, however, address the question of how control commands are to be determined for adjustable optical elements, so that a desired target illumination angle distribution is achieved in the mask plane.